We are living in a gaseous world and the type of gases surrounding our everyday life, for example in where we live, work or play, is vital to our well-being, safety, and even our very survival. Exposure to prolonged insufficient oxygen levels (˜15% or less) can make us very sick or might even be fatal to us at times.
Too much water vapor in the air surrounding us, especially when the temperature is very high (>90° F.), can make us very uncomfortable or seriously ill. For older folks, exposure to high humidity and very high temperature for prolonged periods of time can even be fatal. Unchecked exposure to, or unintentional breathing of, toxic gases above a certain high concentration level such as Carbon Monoxide (70-400 ppm), Hydrogen Sulfide (50-200 ppm), Formaldehyde etc., (>50 ppb), to name just a few, is extremely hazardous to one's health and often leads to unexpected deaths.
In order to prevent accidental or unintended exposure to unsafe levels of gases, humans have long devised, literally from centuries ago until today, various means of detecting all manners of gases, whether they are actually harmful to them or not. Today one can classify all the significant and still prevalent gas measurement techniques developed to date into two broad categories, namely, interactive and non-interactive types. Among the interactive types are electrochemical fuel cells, tin oxide (SnO2) sensors, metal oxide semiconductor (MOS) sensors, catalytic (platinum bead) sensors, photo-ionization detectors (PID), flame-ionization detectors (FID), thermal conductivity sensors etc., almost all of which suffer from long-term output drifts, short life span and non-specificity problems. Non-interactive types include Non-Dispersive Infrared (NDIR), photo-acoustic and tunable diode laser absorption spectroscopy (TDLAS) gas sensors. Up and coming non-interactive techniques advanced only during the past two decades include the use of the latest micro electromechanical technologies such as MicroElectronic Mechanical Systems (MEMS) and the so-called Nano-technology. However, probably a few more years have to pass before the potential of these new non-interactive type gas sensors is fully obtainable.
With so many gas detection techniques available over the years, one could easily be misled to believe that gas sensors today must be plentiful and readily available to people to avoid harmful exposure to unhealthy or toxic gases. Unfortunately, at the present time, this is far from being the truth. The reasons are constraints arising from sensor performance and sensor cost. As a result, gas sensors today are deployed for safety reasons only in the most critical and needed circumstances. An example can be cited in the case of the kerosene heater. A kerosene heater is a very cost effective and reliable appliance used all over the world for generating needed heat during the winter months. However, it can also be a deadly appliance when used in a space where there is inadequate ventilation. In such a situation, as oxygen is being consumed without adequate replenishment, the oxygen level in the space can drop to a point (<15 volume %) where it is injurious or even deadly to inhabitants if they are not adequately forewarned. Therefore, by law or code most every worldwide locales where kerosene heaters are used, this appliance must be equipped with a low oxygen level alarm sensor. Unfortunately, the lowest unit cost for such a sensor available today is only of the electrochemical type. Even so, the unit cost is still in the range of US $15-20. Furthermore, such a sensor is not even stable over time and has a life span of only 3-5 years, far shorter than the 15-20 years expected for the kerosene heater.
In short, gas sensors available to the public today for use to guard against accidental or unintended exposure to unhealthy or toxic gases are very limited and are invariably inadequate taking into consideration both performance and unit sensor cost. This situation will continue to prevail if no breakthrough gas sensor technology is forthcoming.
Although the Non-Dispersive Infrared (“NDIR”) technique has long been considered as one of the best methods for gas measurement, at least from the performance standpoint as being highly specific, sensitive, relatively stable, reliable and easy to maintain and service, it still falls far short of the list of sensor features optimally or ideally needed today. This list of the most desirable gas sensor features will be briefly described below.
The first and foremost desirable feature of a gas sensor to be used for alerting people when they are faced with harmful or toxic gases exceeding a level limit is output stability over time or what is sometimes referred to as having a thermostat-like performance feature. This feature reflects, in essence, the reliability or trust in the use of the sensor. The experience of most people in the use of a thermostat at home is that they are never required, once the sensor is installed, to re-calibrate the sensor and its output stays accurate over time. Such is not the case for gas sensors at the present time. As a matter of fact, no gas sensor today has this desirable feature of having its output stay drift-free irrespective of any measurement technology used for its design and construction.
Gas sensors today have to rely upon periodic re-calibration or output software correction in order to be able to stay drift-free over time. Most recently, the present author advanced in U.S. patent application Ser. No. 12/759,603 a new NDIR gas sensing methodology which renders to first order the output of an NDIR gas sensor designed using this methodology virtually drift-free over time without the need for any sensor output correction software or periodic re-calibration. Thus, it appears hope now exists for the first time for achieving the first and foremost desirable feature of a gas sensor.
The next most desirable feature of a gas sensor is its sensitivity accuracy or its ability to accurately detect the gas of interest to a certain concentration level (e.g., so many ppb or ppm), even in a temperature or pressure hostile environment. Closely related to this feature is detection specificity, namely the capability of a gas sensor to detect the gas of interest free from any interference by other gases in the atmosphere. Another desirable feature of a gas sensor is its ruggedness or its ability to withstand reasonable mechanical abuse (such as a drop from a height of 4-5 feet onto a hard vinyl floor) without falling apart or becoming inoperable. A further desirable feature of a gas sensor is its size and weight, since it is generally desired that such a sensor be small and as light-weight as possible. Yet another desirable feature of a gas sensor is its operating life expectancy (and it is desirable that it have a life span of 15-20 years, or more). Last, but certainly not least, it is desirable that the unit cost of a gas sensor be low enough that it can be affordably applied anywhere. Other than sensor output stability over time, a low unit cost feature is by far the most important desirable feature of a gas sensor, but is also the most difficult to overcome.
It is amply clear that none of the gas sensors available for purchase and use by the general public today meet all of the desirable performance and low unit cost features outlined above. Nevertheless, the long-felt need to have such gas sensors available has not diminished one single iota. The object of the current invention is to advance a novel design for NDIR gas sensors, building upon U.S. patent application Ser. No. 12/759,603 by the present author, such that all the desirable features in sensor performance and sensor unit production cost, hitherto unavailable to the general public, can be attained.